fix: link field relationship should be empty if custom type

[creative-andrew]

How to reproduce:

1. Go to the demo site and create a new post.

2. Add in the Page layout a content block.

3. Add a link to the content block first using Internal link as type .

4. Save and the response should be like this including the corresponding reference object.

   "links": [
   {
     "link": {
       "type": "reference",
       "label": "fdfdfd",
       "reference": {
         "value": {
           "id": "64cb7aa12ad37e39db23ecef",
           "hero": {
             "type": "contentMedia",
             "basic": {
   
             },
             "content": {
   
             },
             "contentMedia": {
               "links": [
                 {
                   "link": {
                     "appearance": "text",
                     "type": "custom",
                     "label": "Documentation",
                     "reference": null,
                     "url": "https://payloadcms.com/docs/getting-started/what-is-payload"
                   },
                   "id": "61f9b0479d0e9c0b6424c3e0"
                 }
               ],
               "media": "64cb7aa12ad37e39db23ecd9",
               "embeddedVideo": {
                 "embed": true,
                 "poster": "64cb7aa12ad37e39db23ecd9",
                 "platform": "youtube",
                 "videoID": "bxWsZTtqs80",
                 "aspectRatio": "56.25"
               }
             },
             "contentSidebar": {
   
             },
             "fullscreenBackground": {
   
             },
             "quickNav": {
   
             },
             "fullscreenSlider": {
   
             }
           },
           "author": "64cb7aa12ad37e39db23ecd6",
           "parent": "64cb7aa12ad37e39db23ecdb",
           "meta": {
   
           },
           "_status": "published",
           "createdAt": "2023-08-03T10:00:02.007Z",
           "updatedAt": "2023-08-03T10:00:03.573Z"
         },
         "relationTo": "pages"
       }
     },
     "id": "64cb85283ae7938606124ba8"
   }
   ],

5. Modify the link to custom type and save. You will notice that the reference is still saved in the field even tho we are using a custom-type link

"links": [
  {
    "link": {
      "type": "custom",
      "label": "fdfdfd",
      "reference": {
        "value": {
          "id": "64cb7aa12ad37e39db23ecef",
          "hero": {
            "type": "contentMedia",
            "basic": {

            },
            "content": {

            },
            "contentMedia": {
              "links": [
                {
                  "link": {
                    "appearance": "text",
                    "type": "custom",
                    "label": "Documentation",
                    "reference": null,
                    "url": "https://payloadcms.com/docs/getting-started/what-is-payload"
                  },
                  "id": "61f9b0479d0e9c0b6424c3e0"
                }
              ],
              "media": "64cb7aa12ad37e39db23ecd9",
              "embeddedVideo": {
                "embed": true,
                "poster": "64cb7aa12ad37e39db23ecd9",
                "platform": "youtube",
                "videoID": "bxWsZTtqs80",
                "aspectRatio": "56.25"
              }
            },
            "contentSidebar": {

            },
            "fullscreenBackground": {

            },
            "quickNav": {

            },
            "fullscreenSlider": {

            }
          },
          "author": "64cb7aa12ad37e39db23ecd6",
          "parent": "64cb7aa12ad37e39db23ecdb",
          "meta": {

          },
          "_status": "published",
          "createdAt": "2023-08-03T10:00:02.007Z",
          "updatedAt": "2023-08-03T10:00:03.573Z"
        },
        "relationTo": "pages"
      },
      "url": "dfddf"
    },
    "id": "64cb85283ae7938606124ba8"
  }
],

Props: @italodr

[JarrodMFlesch]

I think this is a good change, cuts down on the response object/size too.